The long-term goal of this work is to understand, in precise molecular term , how the yeast transcriptional repressor alpha 2 turns off transcription of two set of cell-type specific genes. This protein always acts in combinati n with one of two additional proteins. In alpha cells it cooperates with the GRM protein; in a/alpha cells, it acts with the al protein. These accessor proteins director alpha 2 to two different types of operator and thus enabl alpha 2 to turn off expression of two different sets of genes. Three specif c questions addressed by the proposal are as follows: (1) How do these regulatory proteins act together? (2) How do they reshuffle to create different regulatory activities? (3) How, once bound to its operators, doe alpha 2 carry out repression of its target genes. The basic approach is to use results of simple biochemical and genetic experiments to construct and test molecular models. ONe immediate experimental goal, for example, is a reconstruction and study of the different pairwise combinations of regulators using only purified proteins. A second experimental approach is a reconstruction of transcriptional repression by alpha 2 in vitro and a determination of the step in transcription that is blocked by the action of alpha 2. The proposed studies of alpha 2 should provide a detailed molecular picture showing how cell specialization is maintained in a simple eukaryote, S. cerevisiae. Given the similarity of alpha 2 to cell-type regulators in higher organisms, it is likely that many of the principles developed for alpha 2 will apply in other settings. A basic understanding of the molecul r events underlying cell specialization is essential for understanding how th process can fail, a condition that can give rise to a number of different pathological states.